The control of industrial processes has been a continuing subject of study and innovation on the part of investigators. One aspect of these studies has dealt with control over material dimension in the course of its production. For example, accurate control over the width of weblike cord employed in the manufacture of automotive tires assures a final balanced product capable of meeting the increasingly rigid tolerances mandated by the transportation industry. Similarly, many industries have introduced statistical process control procedures which call for very accurately produced supply materials such as sheet steel. In the latter regard, many steel using industries call for the initial production of rolled steel exhibiting accurately controlled widthwise dimensions. Periodic width checks on the part of plant personnel using manually manipulated measuring devices are no longer adequate to maintain dimensional tolerances now demanded. To achieve such dimensional control, accurate measurements generally must be carried out on a non-contacting and substantially continuous basis. The resultant process control not only achieves production accuracy, but also minimizes material waste otherwise resulting from continued production under out of tolerance conditions.
One measurement system which has found substantial acceptance in industry is marketed under the trademark "SCAN-A-LINE". This system employs a linear array of light emitting diodes positioned on one side of material such as a web or sheet or the like moving within a production process. The diodes of this array are illuminated in a scanning sequence having a stable time base, for example at a 20 KHz rate developed by a quartz crystal oscillator. Positioned above the moving material under production and opposite the associated diode array is a tuned photoresponsive receiver which reacts to the illumination emanating from those diodes which are unblocked or partially blocked at the edges of the moving material. Because diode arrays available in the marketplace typically are spaced at 1/10th inch and/or 1 mm center-to-center, the receiver and associated control system are called upon to carry out an extrapolation process to develop mandated accuracy in locating the position of an edge. This extrapolation is based upon the observation that each LED in the emitting array produces a cone of light and the light cones from adjacent LED's overlap each other in the light path to the photoresponsive receiver. An edge of a product being measured blocking the light path from the emitting diodes to the receiver will attenuate the light from more than one diode. For example, one LED might be attenuated by 12%, while the next adjacent diode is attenuated by 47% and still the next adjacent diode is fully blocked. The signal processing procedure carrying out extrapolation takes samples of the peak amplitude of the light received in sequence from the partially blocked and unblocked LEDs and develops therefrom a time based stairstep light output pattern representing a scan across the edge which, in effect, is smoothed through the utilization of a low pass filtering stage. The edge position of the material being observed then is defined as the time equivalent point on this smooth curve signal where the voltage drops to one-half of the peak LED signal amplitude.
The receiver component utilizes a solar cell form of silicon photodetector which has a length parallel to the array of diodes of 20 mm. This photoresponsive device is positioned above the diodes at what is termed a "stand-off distance" which preferably will be twice the length of the diode array itself. In practical applications, often constraints imposed by the industrial process will cause this distance to vary. Heretofore, in producing the LED emitting arrays, which may vary, for example, from 10 to 400 diodes for any of a variety of industrial applications, sub-components of the arrays, for example having 10 diodes, are evaluated for the purpose of developing a reasonably uniform light output intensity. In this regard, it has been observed that LED light output from an average purchased lot of the device varies by 30% or more.
As the requirements for continuous non-contacting dimensional evaluation of materials in production increases and additional applications of such measuring systems, for example in edge location and guiding are called for by industry, a concomitant need has developed to further increase the accuracy of this practical and relatively lower cost measuring system. Further, a desire has been expressed for expanding the utility of the instant system to carry out edge evaluations of materials which are not opaque, for example such as clear plastics and glass. Edge location for these materials assumes more and more importance in such fields as blown film formation and the like where the diameter of an inflated tube relates directly to the end product thickness. However, the attenuation of light emanating from a diode array on the part of such materials may be quite low under fully "blocking" conditions, for example in the 7% to 10% range. Generally, the only attenuation of light is the result of surface reflection or light refractive phenomena.